Flat panel displays are of immense importance in electronics. In current developments, Active Matrix Liquid Crystal Displays (AMCLD) are beginning to challenge the dominance of Cathode Ray Tube (CRT) technology. AMCLD devices are non-emissive and require complex lithography. Filters and matching spectral backlights are required to produce color. However, there are many light losses and inherent complexity in AMCLD devices because of the non-linear nature of liquid crystal materials. These result in a display that is less bright than a CRT, with a smaller color gamut and poorer viewing angle and contrast. Also, due to the non-emissive nature of the display, inefficient use of input electrical power is made, often with well over 70% of the energy being lost as non-useful energy.
Field emission displays based on conventional “Spindt tip” technology promised a solution to many flat panel display problems. Field emission displays (FED=s) are essentially flat cathode ray tube (CRT) devices. However, rather than one electron gun firing electrons at a phosphor on a screen through a shadow mask, the FED has tens or hundreds of individual tips in each display pixel. The tips are known as Spindt tips, after Cap Spindt. The process of fabrication relies on defining a pattern of holes in a gate metal by photolithography. An underlying insulator is then etched in an isotropic wet etch that “undercuts” leaving a well beneath the metal. A sacrificial layer, usually nickel, is then evaporated onto the surface at an oblique angle to ensure the well is not filled. The emitter material, usually tungsten or molybdenum, is then evaporated through the holes into the well. As the evaporated metal builds up on the surface on the sacrificial layer, it closes the hole as the thickness increases, and has the effect of providing an emitter tip in the well. The top layer of metal is then removed by etching the sacrificial layer, leaving the tip, the well, and the original gate material. This forms the backplate of Spindt tips. A top plate containing a patterned phosphor is then placed relative to the backplate using spacers. The final device may then be evacuated to allow the electrons emitted a long mean free path. However, the device may alternatively be provided with an emission layer disposed between the Spindt tips and the top plate.
The principle of field emission from microtips is well understood and is governed by Fowler-Nordheim tunnelling. The emission current, and therefore the brightness of the display, depends only on the current density, the number of tips and their sharpness, i.e.I=JFNnαwhere, n=number of tips, α=the tip sharpness, and JFN=the Fowler-Nordheim tunnelling current density. The tips will provide a sharp electron source that will provide hot electron injection into an emission layer comprising, for example, an electro-luminescent material.
Unfortunately, the extreme complication in fabrication has limited the use of this technology. Additionally, crystal silicon emitters are limited by the wafer size.
Further, when such devices are provided with a polymeric electro-luminescent emission layer, the emission layer is often fabricated by applying the polymer material to a preformed backplate using a relatively low cost technique such as spin coating, dip-coating or solution casting. However, the polymers which allow such techniques to be performed easily are typically low molecular weight organic materials, often having chemical or morphological instability, and thus the stability of the device as a whole is reduced.
An alternative fabrication technique of using vacuum deposition to dispose the emission layer on the backplate means that the fabrication cost of the device is relatively high.
Other thin-film materials may also be used for field emission. Carbon is the main contender with diamond, diamond like carbon and carbon nano-tubes also being suitable. The use of diamond seemed a good choice, although it is difficult to fabricate and also the mechanism of a supposed negative electron affinity which diamond was claimed to have has now been refuted.
It is the case in some of such known devices that the light emission layer is formed by simply mixing fluorescent dye into the polymer. Such mixes also suffer from morphological instability by phase separation which restricts the application of fluorescent dye mix polymer matrices at high dye concentration.
Accordingly, improvements on the prior art are desired, and these are now provided by the present invention.